Solid-state organic light emitting diode (OLED) image display devices are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the device age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials may age at different rates, causing differential color aging and a display whose white point varies as the display is used.
Referring to FIG. 2, a graph illustrating the typical light output of a prior-art OLED display device as current is passed through the OLEDs is shown. The three curves represent typical change in performance of red, green and blue light emitters over time. As can be seen by the curves, the decay in luminance between the differently colored light emitters is different. Hence, in conventional use, with no aging correction, as current is applied to each of the differently colored OLEDs, the display will become less bright and the color, in particular the white point, of the display will shift.
A variety of methods for measuring or predicting the aging of the OLED materials in displays are known in the art. For example, U.S. Pat. No. 6,456,016 issued Sep. 24, 2002 to Sundahl et al., titled “Compensating Organic Light Emitting Displays” relies on a controlled reduction of current provided at an early stage of device use followed by a second stage in which the display output is gradually decreased. U.S. Pat. No. 6,414,661 entitled “Method And Apparatus For Calibrating Display Devices And Automatically Compensating For Loss In Their Efficiency Over Time” issued Jul. 2, 2002 to Shen et al, describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes (OLEDs) in an OLED display device, by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. U.S. Published Patent Application No. 2002/0167474 “Method Of Providing Pulse Amplitude Modulation For OLED Display Drivers” published Nov. 14, 2002 by Everitt describes a pulse width modulation driver for an organic light emitting diode display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver may receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics.
U.S. Pat. No. 6,504,565 titled “Light-Emitting Device, Exposure Device, And Image Forming Apparatus”, issued Jan. 7, 2003 to Narita et al describes a light-emitting device which includes a light-emitting element array formed by arranging a plurality of light-emitting elements, a driving unit for driving the light-emitting element array to emit light from each of the light-emitting elements, a memory unit for storing the number of light emissions for each light-emitting element of the light-emitting element array, and a control unit for controlling the driving unit based on the information stored in the memory unit so that the amount of light emitted from each light-emitting element is held constant.
JP 2002/278514 A titled “Electro-Optical Device” and published Sep. 27, 2002 by Koji describes a method in which a prescribed voltage is applied to organic EL elements by a current-measuring circuit and the current flows are measured. A temperature measurement circuit estimates the temperature of the organic EL elements.
All of the methods described above change the output of the OLED display to compensate for changes in the OLED light emitting elements. However, it is preferable that any changes made to the display be imperceptible to a user. Since displays are typically viewed in a single-stimulus environment, slow changes over time are acceptable, but large, noticeable changes are objectionable. Since continuous, real-time corrections are usually not practical because they interfere with the operation of the OLED display, most changes in OLED display compensation are done periodically. Hence, if an OLED display output changes significantly during a single period, a noticeably objectionable correction to the appearance of the display may result.
It is also true that in any real system, measurement anomalies may occur due to environmental or system perturbations or noise that do not reflect the actual situation. Corrections in response to such anomalies are undesirable and may result in damage to the system or may degrade display performance. Manufacturing processes used to make OLED displays also exhibit variability that affects the performance of the display and this manufacturing variability needs to be accommodated in any practical aging correction method.
Referring to FIG. 3, prior art systems providing aging compensation to OLED displays typically include a display 30 for displaying images. The display 30 is controlled by a controller 32 that receives image or data signals 34 from an external device. The image or data signals 34 are converted into the appropriate control signals 36 using conversion circuitry 38 within the controller 32 and applied to the display 30. A performance attribute of the display, for example the current or voltage within the display 30, is measured and a feedback signal 40 is supplied through a measurement circuit 42 and provided to the controller 30. The controller then uses the measured feedback signal 40 to change the control signals 36 to compensate for any aging detected in the display 30.
The measurement circuit 42 may be incorporated into the display 30, into the controller 32, or may be a separate circuit 42 (as shown). Likewise, the feedback signal may be detected within the display (as shown) or measured externally by the controller 32 or some other circuit. For example, the luminance of the display 32 may be measured by an external photo-sensor or camera or be detected by photosensors on the display itself.
In some prior art embodiments, the feedback signal 40 is not produced by the display 30, but is produced by analyzing the control signals 36 input to the display 30. For example, a useful feedback signal known in the prior art is the accumulation of current provided to the display 30. Since aging depends on total current passed through a display, a measurement of the accumulated current can be used to predict the aging of the display 30. Alternatively, the luminance signal sent to the display 30 as part of the control signals 36 may be accumulated over time to provide the feedback signal 40. A knowledge of the intended luminance of the display 30 can be used to predict aging and then the effects of aging can be compensated. Although a continuous correction of aging is possible in some of these configurations, corrections are often applied periodically so as not to interfere with the use of the device.
It is also the case that some environmental factors, for example temperature of operation, length of operation, and time since previous operation all contribute to the efficiency of the display. It is difficult to accommodate all environmental factors in a correction scheme. Therefore, it is important to provide corrections that are robust in the face of unanticipated environmental variables. The methods shown in the prior art do not address these environmental variables.
There is a need therefore for an improved aging compensation method for organic light emitting diode displays.